London's Super Sewer Cut Storm Sewage Overflows Into the Thames by About 86% in Its First Full Year

For more than a century and a half, heavy rain in London has meant raw sewage pouring into the River Thames, because the Victorian sewers simply overflowed. A new 25-kilometre tunnel built to catch that overflow is now showing what it can do. The first annual Tideway Benefits Report, published on 13 October 2026, finds that storm overflows into the tidal Thames fell by about 86% compared with the period before the tunnel.

The Thames Tideway Tunnel, which opened in 2025, captures and stores storm sewage that would previously have spilled into the river during heavy rain. According to the report, annual overflow volumes fell to just under 3 million cubic metres in 2025, from a pre-Tideway average of more than 21 million. The tunnel also kept about 84 tonnes of sewage-derived litter, such as wet wipes, sanitary products and plastics, out of the Thames that year, with around 130 tonnes intercepted across the wider tunnel system.

There are signs of benefits for people and wildlife. Days with an elevated health risk linked to overflows fell from an average of 75.7% of days to 10.4% in 2025. Foreshore monitoring by the charity Thames21 found that sewage-related items fell from 30% of litter before the tunnel to 18% in 2025, and the report recorded only one dissolved-oxygen depletion event linked to overflows, creating better conditions for fish. The findings complement the Zoological Society of London's State of the Thames work, in a river that was declared biologically dead in the late 1950s.

"This report shows that the Thames Tideway Tunnel is already delivering real benefits for the River Thames," said Tess Fayers, director of waste and bioresources at Thames Water. Henrietta Gibb, director of impact and engagement at Tideway, said the results show "the profound difference" the tunnel is making, and Alison Debney of ZSL said investment in water infrastructure "delivers tangible improvements in water quality, benefiting both wildlife and public health."

The figures need some care. The 86% is based on modelled volumes rather than measured discharges, and 2025 was an unusually dry year, although the report argues the drop far exceeds what rainfall alone would explain. Operation of the tunnel is now passing from Tideway to Thames Water, which, as Positive News notes, still faces wider pollution and performance challenges. Even so, the first full year suggests London's long-awaited super sewer is working.